The present invention relates to a double chain linear actuator and, in particular, to a double chain linear actuator having helically winding chains.
One measure of a linear actuator is the ratio of extended length to retracted length. For most linear actuators this ratio is 1.8 to 1. Although, there are linear actuators, such as scissor lifts and telescoping hydraulic jacks, in which the ratio of extended length to retracted length exceeds 3 to 1. This allows such linear actuators to provide a relatively large amount of movement while being positioned in a relatively small space. However, these linear actuators typically have a poor side load stiffness and/or require oil. For example, telescoping hydraulic jacks do not have particularly good side load stiffness and require oil. Accordingly, telescoping hydraulic jacks cannot be used in applications which require a good side load carrying capacity or where the use oil is not permissible.
It is known to provide linear actuators having extending and retracting chains, with inter-connected links, as disclosed in U.S. Pat. No. 5,271,182 to Greisner et al., and European Patent Application Number 1,484,465 to Soerensen. These prior art linear actuators have a good side load stiffness and do not require oil. However, they do not have a large extended length to retracted length ratio because a planar chain track, or chain path, is required within the actuator housing to guide the chain as it is being displaced. As a result, these types of linear actuators are typically used in applications where limited movement is required, or where there is no requirement for the linear actuator to be stored in a relatively small space.